Measurement of blood pressure by stethoscopic method employs the steps of raising the pneumatic pressure in a cuff to a predetermined level (maximum blood pressure plus .alpha.) and then progressively decreasing the pressure (at a rate of 2 to 4 mmHg per cardiac cycle).
If the measurement is conducted during rising of the pneumatic pressure instead of falling of the same, pulsation of the pressurizing pump and sounds generated by movable parts such as valves and the diaphragm of the pump are undesirably picked up by a microphone used for detecting Korotkoff sound or pressure sensor used for sensing pulsating pressure change in the cuff Thus, noises are undesirably input to the microphone or the pressure sensor Usually, the frequency bands of these noises overlap that of Korotkoff sound so that it is difficult to discriminate the Korotkoff sound from such noises Therefore, automatic sphygmomanometer, capable of measuring blood pressure during rise in the cuff pressure, is not popular insofar as a pressurizing means other than a compressed gas cylinder is used as the source of pressurized air.
Two methods are available for enabling a rise in the cuff pressure without noise: namely,
(a) a method in which air supplied from a pressurizing pump is temporarily stored in an air reservoir and, after the pump is stopped, compressed air is released from the air reservoir into the cuff so as to establish a cuff pressure; and
(b) a method in which air from the pressurizing pump is made to flow through a flow resistor so as to remove operation noise produced by the pump.
The first-mentioned method (a) has a disadvantage in that the size of the air reservoir must be increased which makes it difficult to install the reservoir on the sphygmomanometer, while the second-mentioned method (b) requires a pump having a large pumping capacity in order to overcome the resistance produced by the flow resistor. Both the methods (a) and (b) cannot establish the required cuff pressure in a short time. Namely, if the measurement is not conducted during the rise of the cuff pressure, it is permitted to supply air at high rate because noises produced during the air supply does not cause any problem. However, when the measurement is conducted during rise of the cuff pressure, the supply of air has to be conducted gently in such a manner as to avoid generation of noise. In addition, when the flow resistor is made of a single orifice, there is a risk that the diameter of the orifice port is progressively decreased or clogged with dust.